The present invention relates to a flat woven papermaker""s forming fabric having a paper side layer and a machine side layer interconnected by weft binder yarns. Each weft binder yarn in sequence interweaves with the paper side layer warp yarns in segments of the weft yarn path so as to complete the paper side layer weave pattern, and to contribute to the properties of the paper side surface of the paper side layer. Each weft binder yarn interlaces with a machine side layer warp yarn, to bind the paper and machine side layers together. Within the overall fabric weave pattern, the number of weft yarns between pairs of weft binder yarns in the paper side layer is irregular.
Flat woven papermaker""s forming fabrics in which so-called xe2x80x9cintrinsicxe2x80x9d weft binder yarn pairs are used to interconnect the weave structures of the paper and machine side layers are well known. Various arrangements have been described, for example by Wilson, U.S. Pat. No. 5,518,042; Vohringer, U.S. Pat. No. 5,152,326; Quigley et al., U.S. Pat. No. 5,520,225; Ostermayer et al., U.S. Pat. No. 5,542,455; Wright, U.S. Pat. No. 5,564,475; Wilson, U.S. Pat. No. 5,641,001; Ward, U.S. Pat. No. 5,709,250; Seabrook et al., U.S. Pat. No. 5,826,627; and Wilson, U.S. Pat. No. 5,937,914. Many others are known.
One feature that is common to all of these known forming fabric designs is that they are essentially xe2x80x9cregularxe2x80x9d and xe2x80x9cevenxe2x80x9d. The spacing of the intrinsic weft binder yarn pairs is regular there being the same number of paper side layer weft between each binder yarn pair, and the interlacing points of each member of the intrinsic weft binder pair into the machine side layer are evenly spaced in both the machine direction and cross-machine direction, within the fabric weave pattern repeat. Thus there is always one, or two, or even three wefts in between each intrinsic weft binder yarn pair.
These references also teach, for example in Seabrook et al. and in the two Wilson disclosures, that the two members of a weft binder yarn pair can occupy a single weft path in the paper side layer such that when one of the members interweaves into the paper side layer thus occupying one segment of the weft path, the other interlaces with a warp in the machine side layer. These disclosures also teach that there can be none, one, two, or three paper side layer warp yarns in between successive segments of the weft path.
As used herein, the following terms have the following meanings.
The term xe2x80x9cweft binder yarnxe2x80x9d refers to each yarn of a pair of yarns which together occupy a single unbroken weft path in the paper side layer, and which separately interlace with a machine side layer warp yarn.
The term xe2x80x9cinterweavexe2x80x9d refers to a locus at which a yarn forms at least one knuckle with another yarn in the paper side layer.
The term xe2x80x9csegmentxe2x80x9d refers to a locus at which a weft binder yarn interweaves with at least one paper side layer warp.
The term xe2x80x9cinterlacexe2x80x9d refers to a point at which a yarn wraps about another yarn in the machine side layer to form a single knuckle.
The term xe2x80x9cfloatxe2x80x9d refers to that portion of a yarn which passes over, or under, a group of other yarns in the same layer of the fabric without interweaving or interlacing with them. The associated term xe2x80x9cfloat lengthxe2x80x9d refers to the length of a float, expressed as a number indicating the number of yarns passed over. A float can be exposed on the machine side or paper side of each of the paper side layer and the machine side layer. The term xe2x80x9cinternal floatxe2x80x9d thus refers to a float exposed between the two layers, either on the machine side of the paper side layer, or on the paper side of the machine side layer.
The terms xe2x80x9cregularxe2x80x9d and xe2x80x9cirregularxe2x80x9d refer to the number of wefts in between successive weft binder yarns in the paper side layer within the fabric weave pattern repeat. In a regular fabric, the number of intervening wefts is constant; in an irregular fabric the number of intervening wefts is not constant.
The terms xe2x80x9csymmetryxe2x80x9d and xe2x80x9casymmetryxe2x80x9d, and the associated terms xe2x80x9csymmetricalxe2x80x9d and xe2x80x9casymmetricalxe2x80x9d, refer to the shape of the path occupied by a weft binder yarn as it exits the paper side layer, interlaces with a machine side layer warp, and returns to the paper side layer. The path is symmetrical when the interlacing point is located substantially at the middle of the path.
The terms xe2x80x9cevenxe2x80x9d and xe2x80x9cunevenxe2x80x9d refer to the location of the interlacing points between a weft binder yarn and a machine side layer warp in the machine side layer within the fabric weave pattern repeat. In an xe2x80x9cevenxe2x80x9d fabric the points are all the same distances apart in each of the machine direction and the cross machine direction and form a coherent pattern; in an xe2x80x9cunevenxe2x80x9d fabric the points are not necessarily all the same distances apart in the machine direction and do not form a coherent pattern.
The notation such as 3/2, for example, in reference to a fabric design refers to the number of warp, or machine direction yarns, over and under which a weft, or cross machine direction yarn, floats within the weave pattern. Thus 3/2 means that a weft yarn floats over three warp yarns and then under two warp yarns within the weave pattern.
Prior to the present invention, the basic approach in fabrics of this type has been to limit the designs chosen for each of the paper side layer and machine side layer to those which were compatible for interconnection with each other. For the two chosen designs to be compatible, two criteria were considered to be important.
First, it must be possible to weave the complete fabric incorporating the designs chosen for the paper side layer and the machine side layer, and including the weft binder yarns which interconnect the two layers together, on one loom. Generally, the number of sheds required to weave the machine side layer when divided by the number of sheds required to weave the paper side layer is an integer, typically 1, 2 or 3. Occasionally, this ratio will be a fraction, such as xc2xd, when a 3-shed machine side layer design is combined with a 6-shed paper side layer design. In general, the number obtained by dividing the higher shed number by the lower one will be an integer.
Second, the paper side layer and machine side layer weave designs must provide internal weft floats (paper side layer) and internal warp floats (machine side layer) which can be interlaced to interconnect the two layers without creating any significant stresses which will distort the planarity of either or both layers. As noted above, this approach resulted in fabrics which are both regular and even. It was also believed that other properties of a forming fabric, such as planar fibre support and wire marking, would be adversely affected if the weft binder pairs were irregularly spaced.
It was generally believed that these limitations would maximise fabric stability, reduce or even eliminate sleaziness (the movement of one of the two layers relative to the other) and reduce the occurrence of fabric delamination caused by both internal and external abrasion of the weft binder yarns.
It is thus apparent that a great deal of experimental and design effort had to be expended in order to find compatible combinations of paper and machine side layer weave designs capable of interconnection by means of weft binder yarns, because the number of compatible paper and machine side layer weave design combinations available for use in forming fabrics of this type has been restricted by the criteria noted above.
This invention is based on the discovery that regularity is not a necessity in forming fabrics of this type. From this it follows that weft binder yarn pairs can be irregularly arranged in the paper side layer, so that within the weave pattern repeat the number of weft between each weft binder yarn pair is not always the same. Since the locations of the internal floats in the weft binder yarns within the paper side layer pattern repeat will determine the interlacing locations, it also follows that the interlacing points in the machine side layer can be selected so as to match the requirements of the paper side layer weave design and need not always be evenly arranged. Conversely, it is also possible to select the machine side layer weave design, then select the paper side layer and then select the interlacing points. It has been discovered that under these conditions it is possible to choose the interlacing locations so that out of plane stresses can be at least reduced, if not substantially eliminated. By introducing irregularity into the paper side layer weave pattern repeat, a much broader range of paper side layer and machine side layer design combinations becomes available, because the fabric designer now has greater freedom to select appropriate paper side layer and machine side layer interlacing point locations, based on the paper side layer and machine side layer weave designs.
In the fabrics of this invention, internal weft floats are provided in the paper side layer, and internal warp floats are provided in the machine side layer. During weaving, these floats are interlaced as desired within the confines of the designs chosen for each of the two layers. There are three parameters which determine the fabric weave pattern. First, the paper side layer weft binder yarn internal float should be as long as possible. Second, the path of the weft binder yarn internal float should be as symmetrical as possible about the interlacing point with the machine side layer internal warp yarn float. Third, in order to protect the weft binder yarn from abrasion, the interlacing point should be as close as possible to the middle of the machine side layer internal warp yarn float.
A second concept used in this invention is that all of the paper side layer weft yarns are substantially the same size. Although some are doubled as weft binder yarn pairs, only one pair member at a time occupies each segment in the unbroken weft path and therefore all of the weft binder yarns contribute to the properties of the paper side layer of the fabric.
Within these broad constraints, it is possible to create a forming fabric in which the weft yarns chosen as weft binder yarns are irregularly spaced.
It is thus apparent that the interlacing locations of the paper side layer and machine side layer internal floats in the fabrics of this invention should be chosen with some care. The limitation on both of these floats appears to be that each should be as long as is reasonably possible within the constraints of the two weave designs. For example, in its path in between the two layers, the paper side float has essentially a xe2x80x9cVxe2x80x9d shape: as the float length increases, the V is flattened reducing the out of plane stresses imposed on the paper side layer. In a similar way, if the V shaped path is not symmetrical, and the interlacing point is close to one end of the float, or the float is relatively short, any stresses imposed on the paper side layer are increased at the shorter end of the float. Similarly, to maximise the protection of the interlacing point, and remove it as far as is practicable from the machine side layer wear plane, the machine side layer internal float should be as long as possible. The upper limits on these two float lengths cannot be directly determined.
The present invention seeks to provide a papermaker""s forming fabric comprising in combination a paper side layer including a first set of warp and weft yarns, in which the weft yarns include weft binder yarns, interwoven according to a first pattern which provides for internal floats of the paper side layer weft binder yarns, a machine side layer including a second set of warp and weft yarns interwoven according to a second pattern which provides for internal floats of the machine side layer warp yarns, wherein within the fabric weave pattern repeat:
(i) the weft binder yarns in pairs together occupy successive segments of an unbroken weft path within the paper side layer;
(ii) the paper side layer weft binder yarn internal floats interlace with machine side layer internal warp yarn floats; and
(iii) the number of paper side layer weft yarns between the weft binder yarns is irregular.
Preferably, each weft binder yarn interlaces at or near to the midpoint of an internal machine side layer warp yarn float.
Preferably, within the pattern repeat, each machine side layer warp yarn interlaces once with a paper side layer weft binder yarn.
Preferably, the path occupied by each weft binder yarn, as it passes from interweaving with the paper side layer warp yarns in a segment of the paper side layer weft yarn path to interlace with a machine side layer warp yarn internal float and return to interweave with the paper side layer warp yarns in another segment of the paper side layer weft yarn path, is more or less symmetrical about the interlacing point.
Preferably, the machine side layer warp yarn internal float length is at least two, and more preferably is at least three. Most preferably, the machine side layer warp yarn float length is four or more.
Preferably, the paper side layer is woven according to a weave design chosen from the group consisting of: a plain weave, a 2/1 twill, a 2/1 broken twill, a 2/1 satin, a 2/2 basket weave, a 2/2 twill, a 3/1 twill, a 3/1 broken twill, a 3/1 satin, a 3/2 twill, a 3/2 satin, a 4/1 twill, a 4/1 broken twill, a 4/1 satin, a 5/1 twill, a 5/1 broken twill, and a 5/1 satin.
Preferably, the machine side layer is woven according to a weave design chosen from the group consisting of: a plain weave, a 2/1 twill, a 2/1 broken twill, a 2/1 satin, a 2/2 basket weave, a 3/1 twill, a 3/1 broken twill, a 3/1 satin, a 3/2 twill, a 3/2 satin, a 3/3/twill, a 4/1 twill, a 4/1 broken twill, a 4/1 satin, a 5/1 twill, a 5/1 broken twill, a 5/1 satin, and an Nxc3x972N design as disclosed by Barrett in U.S. Pat. No. 5,544,678.
Preferably, the ratio of the number of paper side layer weft yarns to the number of machine side layer weft yarns is chosen from the group consisting of: 1:1, 3:2, 5:3, 2:1 or 3:1, when the weft binder yarns are included, and a pair of weft binder yarns counted as one paper side layer weft yarn.
Preferably, the ratio of the number of paper side layer warp yarns to the number of machine side layer warp yarns is 1:1. Alternatively, the ratio of the number of paper side layer warps to the number of machine side layer warps is 2:1.
Both the machine and paper side layers may be woven according any known weave design which would be acceptable for the intended use of the fabric. However, we have found that the machine side layer should be woven according to a design which provides for an internal warp float length of at least three. Although the principles of this invention are equally applicable to nearly all known designs, they are especially applicable to designs whose machine side layer internal warp float lengths are at least 4 or more. This is because in designs which have frequent machine side layer interlacing locations, and which are woven according to designs which provide float lengths of one (plain weave), or two (2/1 satins, twills or broken twills), although there are a large number of locations that may be utilized for interlacing, none of them provide more than minimal protection for the weft binder yarn. Although the invention can be practiced with a combination of plain weave as each of the paper and machine side layer weave designs, its greatest applicability is to machine side layer weave designs which have longer internal float lengths, where it is possible to find acceptable interlacing locations at which the weft binder yarn can be protected from wear. When the forming fabric is to be used for the manufacture of products such as tissue, towel and the like, machine side layer designs that provide shorter internal warp float lengths, such as a plain weave and a 2/1 twill, can be used.
Preferably, the fabrics of this invention have a 5/1 broken twill machine side layer weave which provides for a float length of five yarns, and one of either a 2/1 twill, satin or plain weave paper side layer design. The 5/1 broken twill machine side layer weave design has been found to be particularly useful, due to its wear resistance and long internal warp float length which allows the interlacing points to be recessed as much as possible.